Computing a Large Refined Catalog of Focal Mechanisms for Southern California (1981-2010): Temporal Stability of the Style of Faulting

Abstract

Using the method developed by Shearer (2002, 2003) termed the HASH method, we calculate focal mechanisms for earthquakes that occurred in the southern California region from 1981 to 2010. When available, we use hypocenters refined with differential travel times from waveform cross correlation. Using both the P-wave first motion polarities and the S/P amplitude ratios computed from three-component seismograms, we determine mechanisms for more than 480,000 earthquakes and analyze the statistical features of the whole catalog. We filter the preliminary catalog with criteria associated with mean nodal plane uncertainty and azimuthal gap and obtain a high-quality catalog with approximately 179,000 focal mechanisms. As more S/P amplitude ratios become available after 2000, the average nodal plane uncertainty decreases significantly compared with mechanisms that include only P-wave polarities. In general the parameters of the focal mechanisms have been stable during the three decades. The dominant style of faulting is high angle strike-slip faulting with the most likely P axis centered at N5°E. For earthquakes of M <2:5, there are more normal-faulting events than reverse-faulting events, while the opposite holds for M >2:5 events. Using the 210 moment-tensor solutions in Tape et al. (2010) as benchmarks, we compare the focal plane rotation angles of common events in the catalog. Seventy percent of common earthquakes match well with rotation angles less than the typical nodal plane uncertainty. The common events with relatively large rotation angles are either located around the edge of the (SCSN) network or poorly recorded.Online Material: Table of HASH parameter settings and figures of 1D velocity models, distribution of earthquakes with different focal-mechanism qualities, comparison of YHS2010 and HS2003 catalogs, and distributions of earthquake focal mechanisms.

Subsurface Geometry of the San Andreas Fault in Southern California: Results from the Salton Seismic Imaging Project (SSIP) and Strong Ground Motion Expectations

“…These phases align approximately with relocated microearthquakes in this area ( Fig. 6c; Lin et al, 2007;Hauksson et al, 2012;Yang et al, 2012). Centered at 6 km depth and model range 22.5 km is a Figure 5.…”

“…For our display of earthquakes, we used hypocenters from the Hauksson et al (2012) catalog, which spans the period of time of 1981-2011, and the focal mechanisms from the Yang et al (2012) catalog for these same hypocenters. We display different subsets of the Yang et al (2012) focal mechanisms for the less seismically active line 4 region than for the more seismically active line 6 region.…”

“…We display different subsets of the Yang et al (2012) focal mechanisms for the less seismically active line 4 region than for the more seismically active line 6 region. For line 4, we chose the three highest-quality categories of focal mechanisms (A, B, and C), excluding the lowest-quality category (D), and we display events of all magnitudes for which focal mechanisms were calculated; this yields 50 events ranging in magnitude from 0.9 to 2.9.…”

A search for evidence of secondary static stress triggering during the 1992 Mw7.3 Landers, California, earthquake sequence

“…Stress change calculations require source geometries that are often not available for small earthquakes, as well as accurate hypocenter locations, such as those obtained via waveform cross correlation. The recent publication of high-quality focal mechanisms by Yang et al [2012] and relocated hypocenters by Hauksson et al [2012] for southern California provides the opportunity to reassess whether secondary stress transfer can be modeled with sufficient accuracy to establish or rule out a triggering effect.…”

The 2015 Fillmore Earthquake Swarm and Possible Crustal Deformation Mechanisms near the Bottom of the Eastern Ventura Basin, California

“…Focal mechanisms were determined with the HASH search method (Hardebeck and Shearer, 2003) as applied by Yang et al (2012). The HASH method returns the most likely set of mechanisms after taking into account various sources of uncertainty, such as several possible 1D velocity models.…”